1. Field of the Invention
The present invention relates to a roll sheet conveying device adapted for use in a recording apparatus or the like, such as a printer, a facsimile equipment or a copying machine, using a sheet, such as a roll of paper or a roll of film.
2. Description of Related Art
Known recording apparatuses arranged to record an image on a roll-shaped recording medium (sheet), such as recording paper or film, while conveying the roll-shaped recording medium are generally arranged as follows.
FIG. 12 shows a sheet conveying path of a recording apparatus of the above-stated kind. Roughly speaking, the recording apparatus is composed of a roll sheet holder 100 arranged to rotatably hold a roll sheet Pr so as to feed the paper of the roll sheet Pr for recording, a sheet conveying part 110 arranged to convey the roll sheet Pr to an image forming part 130, the image forming part 130 arranged to form an image on the roll sheet Pr while holding the roll sheet Pr sent out from the sheet conveying part 110, a sheet delivery part 140 arranged to deliver the sheet after image formation to the outside of the apparatus, and a manual-feed conveying part 120 arranged to feed and convey a manually-fed cut sheet to the image forming part 130.
The roll sheet Pr is rotatably held within the roll sheet holder 100 through a spool 101, and is set within the roll sheet holder 100 with the fore end part of the roll sheet Pr pinched or nipped between paper feed rollers 103 (the fore end of the roll sheet Pr being located at a position A).
A recording operation begins with a sending-out action on the roll sheet Pr. The paper feed rollers 103 first rotate to move the fore end (leader) part of the roll sheet Pr out from the roll sheet holder 100. After that, upon arrival of the fore end part of the roll sheet Pr at the sheet conveying part 110, the roll sheet Pr is conveyed to the image forming part 130 by the conveying force of conveying rollers 111 and the guide of a pair of guides 113 and another pair of guides 114. The roll sheet Pr is then wrapped around the surface of a conveying roller 131 to be further conveyed. The conveying action on the roll sheet Pr comes to a stop a predetermined period of time after the fore end of the roll sheet Pr passes a sheet delivery roller 141.
An image forming action next begins. An image is formed by an image recording head 135 on the roll sheet Pr with the sheet held on a platen 134. Upon completion of the image forming action, the roll sheet Pr is cut by a cutter 142 when a part of the roll sheet Pr where the image is formed reaches a path part located on the lowerstream side of the cutter 142. With the roll sheet Pr thus cut, the image-formed part of the roll sheet Pr (an image bearing sheet) is delivered to the outside of the recording apparatus, for example, by its own weight.
The recording apparatus of the kind mentioned above, however, has presented the following problems. In starting the next image forming process continuously after one image-formed portion of the roll sheet has been cut, the fore end margin of the next image becomes too wide as much as a length C as shown in FIG. 12. Therefore, the roll sheet cannot be considered to be effectively used.
In order to avoid this, the roll sheet Pr must be pulled back to set its fore end near to a nipping part of the sheet delivery roller 141. Further, in the event of an attempt to convey a cut sheet by manually inserting it after image recording on the roll sheet Pr, a portion of the roll sheet wrapped around the conveying roller 131 hinders this attempt. To avoid this, the roll sheet must be pulled back (rewound) at least to a position where its fore end is on the upperstream side of a point of confluence B of the roll sheet path and the manual-feed sheet path.
Further, to facilitate replacing work on the roll sheet, the apparatus of this kind generally has the roll sheet holder 100 arranged to be drawn out from the body of the apparatus. In that case, if the roll sheet holder 100 is drawn out while the roll sheet is in a state of having been moved out from the roll sheet holder 100, the fore end of the roll sheet would remain inside of the apparatus. Under such a condition, the replacing work on the roll sheet would become very difficult.
Then, in order to avoid this trouble, the fore end of the roll sheet must be moved back to the position A within the roll sheet holder 100 before the roll sheet holder 100 is drawn out from the body of the apparatus.
For this purpose, the apparatus must be provided with a roll sheet pullback mechanism. The roll sheet pullback mechanism is arranged to drive and rotate the conveying rollers 103, 111 and 131 in a direction reverse to the direction of sending out the roll sheet.
However, the pullback action of the roll sheet pullback mechanism necessitates a space for allowing a redundantly pulled-back portion of the roll sheet Pr to slacken within the roll sheet holder 100, as indicated with a two-dot-chain line in FIG. 12. That arrangement increases the size and weight of the roll sheet holder 100, and thus increases the size of the apparatus.
It is conceivable to solve this problem by a method of driving the spool 101 concurrently with the pullback action to wind and take up the redundantly pulled-back portion of the roll sheet Pr onto the roll of sheet. The roll-sheet winding speed, however, varies with the roll (coil) diameter of the roll sheet. This problem may be solved by measuring the roll diameter and varying the spool driving speed according to the measured diameter to make the roll-sheet winding speed constant. However, this method necessitates use of a roll-diameter measuring mechanism, a discrete spool driving mechanism and a device for control over the spool driving speed. Such a method not only makes the structural arrangement of the apparatus complex but also causes an increase in cost.
According to another conceivable method which is simpler than the above-stated method, the amount of slack of the roll sheet is measured and the roll sheet is wound up for a predetermined period of time. However, that method also necessitates use of some additional detecting mechanism.
While these methods have the above-stated shortcoming, the problem can be simply solved by setting the spool driving speed at a relatively high speed and by arranging a slip mechanism, within a spool driving mechanism, to have a slip caused to take place by a tension developed when the roll sheet is stretched beyond a prescribed torque. However, it is difficult to set the prescribed torque, because of the following reasons.
When the roll sheet is pulled out from its roll by means of the conveying roller, the roll sheet is caused to obliquely travel even by a slight unevenness of the conveying force of the conveying roller in the direction of the width of the roll sheet and/or by a slight degree of discrepancy between the axis of the roll and that of the shaft of the conveying roller.
In order to prevent the oblique travel of the roll sheet, it is necessary to apply some rotation load (roll sheet brake) to the roll sheet. The greater the braking force, the larger the effect of preventing the oblique travel is. However, in rewinding the roll sheet, the spool must be driven while overcoming the rotation load, so that the setting value of the slip torque of the spool must be increased to greater than the rotation load.
Such an arrangement causes an increase in the driving load, and thus results in unsteadiness in speed and amount of transport of the roll sheet and also in increases in size and power consumption of the motor.
Further, a problem with the spool rewinding method lies in that, in the event of occurrence of a slip between the roll sheet and its roll core, an idle rotation of the roll core takes place to eventually bring about slacking of the sheet within the roll sheet holder.
This slip takes place in cases where the roll sheet is not firmly attached to the roll core with some adhesive or the like at the beginning of winding the roll. However, such cases are not rare. Therefore, to ensure the reliability of the apparatus, the apparatus must be arranged to be capable of adequately coping with such a trouble.
It is an object of the invention to provide a roll sheet conveying device arranged to be capable of always stably performing a rewinding action on the roll sheet, while effectively preventing the roll sheet conveying device from becoming larger in size and complex in structure and from lowering the reliability thereof.
To attain the above object, in accordance with an aspect of the invention, there is provided a roll sheet conveying device, comprising a spool arranged to be rotatable while holding a roll-shaped sheet, sheet conveying means for driving and conveying the sheet, and load generating means for generating a rotation load to act on the spool, wherein, when the sheet is sent out from the spool by driving the sheet conveying means, the rotation load of the load generating means is caused to act on the spool, and when the sheet is rewound onto the spool by driving the spool, the rotation load of the load generating means is prevented from acting on the spool or is caused to lessen to act on the spool.
More specifically, a roll sheet conveying device comprises a spool rotatably supported within a roll sheet holder in a state of holding a roll sheet from inside of a roll core on which the roll sheet is wound, one or more conveying rollers arranged to convey the roll sheet, a driving mechanism arranged to drive and rotate at least one of the spool and the one or more conveying rollers, a rotating direction of the driving mechanism being changeable to change over between conveying modes of conveying the roll sheet in a sending-out direction and in a rewinding direction, and a spool brake mechanism for applying a braking force to the rotation of the spool, wherein, when the roll sheet is sent out from the spool by driving the one or more conveying rollers, the braking force of the spool brake mechanism is caused to act on the spool, and, when the roll sheet is rewound onto the spool by driving the spool, the braking force of the spool brake mechanism is prevented from acting on the spool.
Thus, when the roll sheet is sent out from the spool (i.e., when the roll sheet is unrolled from its roll while causing the spool to rotate through the roll sheet by the driving force of the sheet conveying means), the sheet is prevented from obliquely traveling by causing the rotation load (i.e., the braking force) to act on the spool. On the other hand, when the roll sheet is rewound onto the spool (i.e., when the unrolled sheet is wound back on the roll by the driving force of the spool), an increase of the load on the winding driving is prevented by preventing the rotation load (i.e., the braking force) from acting on the spool.
As for the above-stated change-over, a one-way clutch or the like is employed to connect the spool to the load generating means in sending out the roll sheet and to disconnect the spool from the load generating means in rewinding the roll sheet.
Further, in a case where the roll sheet conveying device according to the invention is provided with a driving mechanism for driving the spool and the sheet conveying means in rewinding the roll sheet, the driving mechanism is arranged to drive the spool at such a speed that a speed at which the roll sheet is taken up onto the spool by the spool and a roll of the roll sheet wound on the spool becomes higher than a driving speed of the sheet conveying means in the rewinding direction throughout the whole process of rewinding the roll sheet. The roll sheet conveying device may be provided further with slip means, such as a torque limiter, arranged to cause the spool to slip over the driving mechanism when the spool receives from the sheet a rewinding resisting force of a value exceeding a predetermined value.
The above-stated arrangement enables the roll sheet conveying device to rewind the roll sheet without slackening it. The size of the roll sheet holder by which the sheet roll is held through the spool thus can be prevented from increasing. Besides, since the load on the sheet rewinding driving action can be prevented from increasing, the latitude of setting a slip generating torque for the slip means is increased and the absolute value of the slip generating torque can be lessened.
Further, in a case where the roll sheet conveying device according to the invention has the spool arranged to hold the core member of the roll sheet, the roll sheet conveying device may be provided with control means which is arranged to detect whether or not a slip takes place between the sheet and the core member in at least one of sending out the roll sheet and rewinding the roll sheet and, upon detection of the slip, to restrain driving of at least one of the sheet conveying means and the spool.
Such control means enables the roll sheet conveying device to effectively prevent the roll sheet from slackening due to a slip taking place between the roll sheet and the core member.
The arrangements according to the invention described above are advantageous particularly for a roll sheet conveying device wherein spools are arranged vertically in a multi-stage manner.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.